It is well established that high levels of dietary fat promotes murine mammary tumorigenesis. Stimulation of mammary tumor growth and progression by dietary fat has been observed in virtually every rodent mammary tumor growth and progression by dietary fat has been observed in virtually every rodent mammary tumor model examined. To date, no one has determined whether high levels of dietary fat can enhance the in vivo growth of normal or carcinomatous human breast tissue. The mechanism by which high levels of dietary fat enhances murine mammary gland tumorigenesis is presently unknown, although a number of mechanisms have been proposed and examined. This proposal will examine a novel mechanism, one that has not been previously proposed or studied, i.e., that high levels of dietary fat enhance mammary tumorigenesis directly by interfering with mammary intercellular communication (metabolic cooperation). Concisely, the specific objectives and methodology of this research proposal are the following: 1. Can high dietary fat enhance the growth of human breast carcinoma cell lines in vivo? Human breast carcinoma cell lines MCF-7, T47-D, MDA-MB-157, and MDA-MB-231 will be transplanted s.c. to athymic nude mice fed diets containing low (0.5%), moderate (4.5%) or high (20%) levels of fat (corn oil). Growth of the carcinomas will be assessed by latency period of carcinoma appearance and weekly tumor volume measurements. 2. Can high dietary fat enhance the growth of normal human breast ductal epithelium in vivo? Slices of normal human breast tissues (biopsy specimens) will be transplanted s.c. to athymic nude mice fed diets containing 0.5%, 4.5% or 20% fat (corn oil). Growth of the ductal epithelium in the human breast tissue grafts will be assessed by autoradiographic analysis of 3H-thymidine incorporation into DNA (labelling index) and whole-mount morphological analysis. 3. Do high levels of dietary fat enhance mammary tumorigenesis by inhibiting intercellular communication (metabolic cooperation)? Metabolic cooperation will be quantitatively assessed in vitro using: 1) HGPRT+ and HGPRT- (mutant) V79 Chinese hamster cells; 2) Lesh-Nyhan (mutant HGPRT-) human fibroblasts and human breast carcinoma cells; 3) 3H-uridine transfer in normal and carcinomatous rat mammary and human breast cell lines. Dietary fatty acids will be added to the culture medium and their influence on metabolic cooperation will be determined using these 3 in votro assays.